1. Field of the Invention
The invention relates to protecting electronic appliances against temperatures that are excessively high or low, to which they might be subjected while they are in operation, or indeed, and above all, while they are not in operation.
The present invention applies very advantageously to secure electronic appliances, in particular postage meters (also referred to as "franking machines"), which contain accounting data representing monetary values (e.g. the postage amounts franked in a postage meter), which values are stored in a memory inside the appliance and must be protected against corruption, whether fraudulent or accidental, due to temperatures outside the temperature range over which the electronic memory of the appliance is guaranteed to retain data correctly.
2. Description of the Related Art
In a postage meter, both the accounting unit (which is implemented around an electronic memory), and also the printer drive mechanism are enclosed in a protected enclosure, access to which is restricted to postal administration employees or to after-sales employees of the manufacturer. For example, such protection is provided by suitable tamper-proof sealing.
Document EP-A-0 099 571 describes such an electronic appliance including an enclosure that is protected both against tampering, and also against electromagnetic interference which might be applied intentionally or accidentally to the circuits of the appliance, and which might corrupt the contents of the accounting memory. A trapdoor is provided to give access to the enclosure, which trapdoor is configured such that once it has been opened, it can no longer be used as a door. Additionally, opening the trap door disables any subsequent operation of the appliance.
Such a postage meter is, therefore, protected against fraud by mechanical tampering or by applying an electromagnetic field which might, for example, enable the contents of the accounting memory to be erased.
Another type of fraud might result from bringing the meter to a temperature that is outside its rated operating temperature range.
For example, by placing the meter in an oven or in a freezer, a dishonest user can easily bring the meter to extreme temperatures at which the accounting memory can be erased, without physically damaging the circuit of the meter.
To prevent such "temperature fraud", it is necessary to detect any attempt to bring the meter to a temperature outside its rated temperature range. This must be done even outside periods when the appliance is in normal use, i.e. it must be possible to detect such fraud even if the meter is switched off or unplugged. It is also necessary to store in a memory the fact that such an attempt has been detected, and to prevent any subsequent use of the appliance.
Attempts to commit fraud by heating the appliance can be detected relatively simply, e.g. by using a well-known technique consisting in placing a thermal fuse in series in the electrical power supply of the meter, the fuse naturally being situated inside the sealed enclosure. For example, if the fuse is calibrated at 70.degree. C., it blows as soon as the inside temperature of the meter exceeds that value, the meter then becoming unusable. To enable the meter to operate again, it is necessary to break the seal on the meter and to change the fuse.
Reliable detection of low temperatures is more difficult.
Document FR-A-2 675 965 proposes a technique of providing a manually-resettable thermostat that is responsive to negative temperatures. Naturally, that component is placed inside the sealed enclosure. More precisely, that component includes two bimetal-disc thermo-sensitive members which control electrical contacts mounted in series, so as to interrupt the power supply to the meter when the temperature leaves the normal operating range, and so as to prevent the meter from operating even when the temperature has returned to that range.
Unfortunately, the protective component proposed by that document is a special component and is therefore costly to industrialize and to produce. Furthermore, that mode of protection, which is solely electromechanical, offers no possibility of adjustment, thus, its accuracy depends entirely on the manufacturing and calibration quality of the bimetal discs. This further increases cost whenever high levels of accuracy, reliability and operating safety are sought.